1. Field of the Invention
The present invention relates to an exposure apparatus that exposes a pattern on a substrate via a projection optical system and a liquid in a state wherein the liquid is filled in at least one part of a space between the projection optical system and the substrate; and a device fabrication method that uses this exposure apparatus.
2. Description of Related Art
Semiconductor devices and liquid crystal devices are fabricated by a so-called photolithography technique, wherein a pattern formed on a mask is transferred onto a photosensitive substrate.
An exposure apparatus used by this photolithographic process includes a mask stage that supports the mask, and a substrate stage that supports the substrate, and transfers the pattern of the mask onto the substrate via a projection optical system while successively moving the mask stage and the substrate stage. There has been demand in recent years for higher resolution projection optical systems in order to handle the much higher levels of integration of device patterns. As the exposure wavelength to be used is shorter, the resolution of the projection optical system becomes higher. As the numerical aperture of the projection optical system is larger, the resolution of the projection optical system becomes higher. Consequently, the exposure wavelength used in exposure apparatuses has shortened year by year, and the numerical aperture of projection optical systems has also increased. Furthermore, the currently mainstream exposure wavelength is the 248 nm KrF excimer laser, but an even shorter wavelength 193 nm ArF excimer laser is also being commercialized. In addition, as well as resolution, the depth of focus (DOF) is also important when performing an exposure. The following equations respectively express the resolution R and the depth of focus δ.R=k1·λ/NA  (1)δ=±k2·λ/NA2  (2)
Therein, λ is the exposure wavelength, NA is the numerical aperture of the projection optical system, and k1 and k2 are the process coefficients. Equations (1) and (2) teach that, when the exposure wavelength λ is shortened and the numerical aperture NA is increased in order to enhance the resolution R, then the depth of focus δ is narrowed.
If the depth of focus δ becomes excessively narrow, then it will become difficult to align the surface of the substrate with the image plane of the projection optical system, and there will be a risk of insufficient margin during exposure operation. Accordingly, a liquid immersion method has been proposed, as disclosed in, for example, PCT International Publication WO99/49504, as a method to substantially shorten the exposure wavelength and increase the depth of focus. This liquid immersion method fills a liquid, such as water or an organic solvent, between the lower surface of the projection optical system and the surface of the substrate, thus taking advantage of the fact that the wavelength of the exposure light in a liquid is 1/n that of in air (where n is the refractive index of the liquid, normally approximately 1.2-1.6), thereby improving the resolution as well as increasing the depth of focus by approximately n times.
Incidentally, inside the chamber of a conventional exposure apparatus (an exposure apparatus for dry exposure), the humidity is lowered and an airflow is generated by an air conditioner, which creates an atmosphere in which liquids tend to vaporize. Accordingly, if it is decided to perform immersion exposure in an environment similar to the inside of the chamber of the conventional exposure apparatus, then there is a possibility that the liquid for the immersion exposure will vaporize, making it impossible to maintain the control accuracy of the temperature of that liquid, the projection optical system (a part of the optical elements) in contact with that liquid, or the substrate. In addition, variations in the temperature of the projection optical system degrade the projected image, and variations in the temperature of the substrate deform (expand and contract) the substrate, creating the possibility that the pattern overlay accuracy will degrade.
The present invention has been made considering such circumstances, and has an object to provide an exposure apparatus and device fabrication method capable of accurately forming the image of a pattern on a substrate when performing the exposure process based on the liquid immersion method. It is another object of the present invention to provide an exposure apparatus and device fabrication method capable of setting and maintaining at a desired temperature the liquid for liquid immersion exposure, and a substrate that is to be exposed.